The present invention relates to radiation detectors and, more particularly, to a new and improved radiation detector of the type which utilizes carriers generated by a radioactive ray in the depletion layer of a semiconductor.
The structure of semiconductor radioactive detectors of the prior art is shown in FIG. 1A and FIG. 1B, which illustrate surface barrier-type detectors, and in FIG. 1C, which illustrates a P-N junction-type detector. In semiconductor radioactive ray detectors a depletion layer is formed by applying a reverse bias to the surface barrier, or P-N junction, but if the resulting reverse leakage current is high, a low signal to noise ratio (S/N) is produced. Such reverse leakage current is classified into a surface leakage current, which flows along the surface of detector, and a volume leakage current, which flows entirely inside the detector. The amount of surface leakage current is determined by the surface condition of the single crystal semiconductor resulting from such factors as the method of manufacture, the nature of surface contamination, the free carriers in the protective surface film, and the carrier trap and interface levels. On the other hand, the volume leakage current consist of a component resulting from diffusion of carriers into the depletion layer and a component resulting from generation of carriers at the trapping centers in the depletion layer.
In a surface barrier-type detector such as shown in FIG. 1A, a protective oxide film 2 is formed at the surface of single crystal silicon plate 1 and a portion of the film 2 having strain due to processing is removed from the surface by chemical etching to provide a window leaving a very thin oxide film 3. A thin metal electrode film 4 is vacuum-deposited over the window and on the thin oxide film 3. However, the very thin oxide film 3 has an unstable composition SiO.sub.x which may change to stable SiO.sub.2 when oxygen in the air migrates through the electrode metal thin film 4 and which also chemically combines at the surface of the single crystal silicon substrate 1, resulting in varations in the ageing characteristic of the detector.
Moreover, an inverted layer is generated at the surface of the silicon substrate 1 because the protective oxide film 2 contains ions and permits a surface leakage current to flow through the path 6 toward an electrode 5 in contact with the silicon substrate 1 at the side opposite to the electrode 3. In addition, charges are generated in the SiO.sub.2 film during irradiation by radioactive rays. For example, continuous radiation of 10.sup.5 R or more causes an increase in surface leakage current. Further, when an N-type substrate is used as the silicon plate 1 and gold is vacuum-deposited as the electrode metal film 4, disadvantages such as weakness to mechanical impact and partial peeling result.
In a surface barrier-type detector such as shown in FIG. 1B, a thin silicon film 7 is coated onto a single crystal silicon substrate 1 by vacuum-deposition with a resistive heating system and, thereafter, an aluminum electrode 4 is formed on the film 7 by the vacuum-deposition method and either gold or aluminum is vacuum-deposited in the same way as an ohmic contact electrode on the opposite side. In this case, since a very thin oxide film is not used, there is no problem of variation in ageing characteristics resulting from stabilization of an unstable oxide but the problem of surface leakage current caused by the surface protection film remains. A detector showing such large reverse leakage current results in decrease in S/N when the diameter becomes large and therefore the detector of this type cannot be used with an area larger than about 1000 mm.sup.2.
In the case of a P-N junction type detector, as shown in FIG. 1C, an impurity layer 8 is generally formed on the single crystal silicon substrate 1 by thermal diffusion or ion implantation and thermal processing for which a temperature of about 800.degree. to 1200.degree. C. is required. However, such high temperature thermal processing causes the volume leakage current to increase because crystal defects are produced or heavy metal ions enter the single crystal semiconductor substrate. Moreover, since oxygen contained in the silicon single crystal changes to a donor during such high temperature thermal processing, the specific resistance of single crystal semiconductor is sometimes lowered. However, when the thermal processing temperature is decreased in order to avoid deterioration of the single crystal material caused by high temperature thermal processing, the following disadvantages result:
(1) The variation in impurity concentration becomes extremely large or the formation of an impurity layer by diffusion becomes impossible.
(2) Introduction of crystal defects by impurity ion implantation is imperfect and, moreover, replacement of the implanted impurities in the lattice position is insufficient.
Accordingly, proper thermal processing at a temperature as low as 800.degree. C. or lower is almost impossible. Therefore, semiconductor detectors are manufactured by conventional high temperature thermal processing which results in a high level of noise, interfering with the characteristics of the device. In this case, since an SiO.sub.2 film 2 is also provided, the surface leakage current following the route 6, or resulting from high radiation levels, increases.
It is an object of the present invention to provide a semiconductor radioactive ray detector which eliminates the above-mentioned disadvantages, does not have variable ageing characteristics, is rigid, and assures good high energy radiation resolution.